Such a setting wheel, used in a so-called precision seed drill, is disclosed in FR-A-No. 2 478 942. In such a sowing machine which ensures the placement of the seeds with a very high precision, it is still more necessary to avoid a displacement of the seed at the moment it is driven in by the setting wheel, due the effect of the sliding speed of the latter. In order to avoid this sliding effect, the hereabove patent proposes to drive the setting wheel in such manner that its peripheral speed is substantially equal to the advancement speed of the sowing machine, and preferably superior by 1 to 2% to said speed. In practice and as proposed in this patent, the setting wheel is rotatably mounted on an axis which is associated in height to the drill coulter which opens the furrow and brings the seeds in the dihedron between the bottom of the furrow and the periphery of said wheel via a channel formed on its rear face and surrounding the lower front periphery of the setting wheel. The wheel is driven in rotation by a transmission device of any known type such as a pinion and chain system from one of the wheels of the sowing machine, in practice the gauging wheel which adjusts the ploughing depth of the coulter of each element, or from the tractor power take-off.
This embodiment has various disadvantages amongst which one can cite the necessity for each setting wheel to use an extra transmission liable to break and to get soiled, and the fact that since the gauging wheel of each element is placed at a distance, in principle rearwardly from the transverse axial line of the setting wheel, the ground irregularities to which the gauging wheel can be faced such as stones, a transverse furrow or a localized ground zone exhibiting an adherence fault have an influence on the depth of the furrow and on the rotation speed of the setting wheel. Moreover, the setting wheels are usually narrow, their width corresponding generally to the width of the furrow and, for preventing the ground from adhering to the periphery and attracting the seeds imbedded in the ground, it is necessary to provide on each wheel a cleaning device which brakes the setting wheel, and this is liable to increase the sliding effect and is remedied by the chain or belt transmission connecting the setting wheel to the gauging wheel which carries the load of the element and therefore is well adhering to the ground.